The invention relates to a joint device, especially a safety joint for a riser extending between a vessel and a subsea installation, which joint comprises two telescoping parts, each defining a fluid channel and interconnected by means that are arranged to be broken at a predetermined axial load.
The invention also comprises a method.
Operations in subsea wells are normally conducted by establishing a closed column connecting the well with a vessel on the surface, thus providing safe access to the well. A column of this kind is usually called a riser or riser system and includes not only the actual pipe but also several other devices which in addition to the actual pipe are necessary for safe access to the well. All operations down in the well are conducted through the riser, which forms a barrier between well fluids and the surrounding seawater. Work is carried out on a “live” well, i.e. the well is open all the way up to the vessel with well fluids at a pressure corresponding to the formation pressure. The riser therefore must be designed to be able to withstand high well pressure. Otherwise an uncontrolled blow-out may cause the riser to be filled with gas from the well, with the result that the pressure inside the riser sinks to almost zero.
The riser system normally comprises a lower riser package LRP with a number of valves for closing down the well, thereby functionally corresponding to a blow-out preventer (BOP). There are also provided an emergency quick disconnect package (EQDP) and a stress joint. At the upper end of the riser, i.e. in the vessel, there is usually provided a surface BOP. In addition the riser may be equipped with a bending member and buoyancy elements, together with any other devices required for operations on a subsea well.
When operations have to be performed on wells located at great depths, a vessel is employed which is kept in the correct position by means of propellers and/or thrusters. Such vessels are called dynamically positioned (DP) vessels. These vessels are highly dependent on all systems working satisfactorily and normal practice requires them to be equipped with several systems as security against the vessel drifting out of position.
During operations from a dynamically positioned vessel, situations may arise where it is necessary to leave the position above the well quickly. This may be controlled, such as when a warning of deteriorating weather conditions makes it necessary to evacuate the position, or uncontrolled, where some of the systems fail and the vessel begins to drift out of position. Such a situation may also occur in the event of sudden bad weather, but particularly in situations where the vessel's systems are not capable of keeping the vessel in the correct position above the well. The
consequences of such a situation may be that the heave compensation system touches the bottom or that the riser assumes an unacceptable angle resulting in loading that exceeds the riser's design load.
Such situations can result in fracture of the riser. In situations of this kind it is important to control the fracture, i.e. to ensure that it occurs at a location where the well's barriers remain intact.
Fracture of the riser may result in damage to the vessel and constitute a risk to personnel as well as causing environmental damage, i.e. spillage of hydrocarbons, hydraulic fluid or the like. This may occur on account of the energy in the tensioned riser and the content of the riser. A complicating factor will be present if the riser has an internal pressure with an unstabilised fluid or a mixture of gas and fluid. The fluid that then flows out of the lower end of the riser will give rise to an upwardly-directed force that attempts to push the riser up in the rig towards the heave compensation, thereby making the situation more unstable. The most extreme consequence is that the riser may be pushed upwards with such force that serious damage is done to the equipment in the vessel and it may even be wrecked. A situation of this kind may also lead to loss of human life.
It is previously known to equip pipes with safety joints that are broken if the pipe is subjected to tension exceeding a predetermined value. This comprises shear pins that are broken when there is tension in the pipe. The position of the fracture can thereby be controlled and it can be located in an area that results in the least possible damage to equipment. However, since the riser has a high internal pressure, dangerous situations may still arise as mentioned above.
It is also previously known to arrange valves in a safety joint in such a way that the fluid channel close when the joint are broken. In WO 2004/055316 are shown and described a working string extending through a riser and down in a well. The working string is arranged to be broken at a predetermined upwardly directed load and comprise valves for closing fluid channels over and under the separation joint respectively. The valves are held open by means of hydraulic pressure but are equipped with pull-back springs which actuate the valve to closing when the hydraulic pressure disappears. By rupture the hydraulic supply pipe will also break in such a way that the pressure in the pipe drops. The disadvantage with this embodiment is that it must be arranged hydraulic connection lines from surface and that a hydraulic pressure must be maintained during the work operations. If this pressure for some reason should be lost, e.g. by failure in the system, the valves will close unnecessary and could cause disturbance or even dangerous situations.
One other example of the prior art is WO 01/86110 which concern a device for disconnecting an upper part of a riser from a lower part which are connected with, or cemented, in a pipe at seabed. The procedure and mechanism for separating the two parts from each other is relative complicated and result in some operational steps which have to be done in correct sequence. The most important moment here is that in order to disconnect the two parts, the tension in the upper part of the riser must be reduced first (or even be compressed (see col. 30, first section)) in order to release the locking mechanisms and afterwards it is put under tension again to pull the upper part of the riser up from the lower part. Thus one has to undertake an active action in order to release the locking mechanism before the disconnection. The disadvantage with this is that in a emergency situation, where the platform drifting off, there can be a situation where it will be impossible to reduce the tension in the riser, because there will simply not be time nor room to carry out a procedure like this. It is self-evident that in a case like this the tension load will increase, and if there are no weakening devices present the riser will actually break.